Example embodiments of the present invention relate to a method for operating a radar sensor, in which a modulation sequence having a number of successive linear frequency ramps having different slopes is cyclically repeated. A received radar signal, which is reflected from an object, is mixed with the emitted radar signal to form an intermediate frequency signal, which is analyzed for each frequency ramp with respect to its frequency spectrum. Peaks occurring in the frequency spectra of the intermediate frequency signal correspond to ambiguity lines in a distance/velocity space. Possible objects are assumed at intersection points of the ambiguity lines. Furthermore, the expected position which the possible objects would have at the point in time of the repetition of the modulation sequence is precalculated.
The present invention further relates to a radar sensor which operates according to the FMCW (frequency modulated continuous wave) principle using a modulation sequence of linear frequency ramps.
Radar sensors are used in motor vehicles, for example, to detect the surroundings of the vehicle and to locate and determine the relative velocity of preceding vehicles or oncoming vehicles. They may be used as independent distance warning systems or also as part of a driver assistance system, for example, for distance-based automatic velocity control (ACC—adaptive cruise control).
The velocity of an object may also be determined with the aid of radar sensors on the basis of the frequency shifts because of the Doppler effect between emitted radar signals and radar signals which are reflected from the object and received. In order to simultaneously obtain information about the distance of the object from the radar sensor, information about the propagation time of the radar signals is additionally necessary. In the FMCW radar method, such propagation time information may be obtained in that the frequency of the emitted radar signal is subjected to a frequency modulation using a linearly changing frequency (frequency ramp).
The received radar signal is typically mixed with a part of the emitted signal to form an intermediate frequency signal. The frequency spectrum of the intermediate frequency signal is analyzed, typically with the aid of a fast Fourier transform. An object detected by the radar system is reflected in the frequency spectrum in a peak at a frequency which is a function of the distance and the relative velocity of the object to the radar sensor.
The distance and velocity of an object cannot be unambiguously determined simultaneously from the frequency of a measured peak, however. Rather, each of the measured peaks represents a plurality of possible combinations of distance d and relative velocity vr of an object. In a distance/velocity space, referred to in short hereafter as d/v space, the plurality of the possible combinations of distance d and relative velocity vr, which correspond to a frequency in the frequency spectrum of the intermediate frequency signal, is shown by a straight line whose slope is a function of the slope of the frequency ramp, i.e., the change in a frequency per unit of time. This straight line is also referred to as ambiguity line.
An unambiguous determination of distance and velocity of an object may be performed by going through a sequence of two successive frequency ramps having different slopes. The frequency of the peak in the intermediate frequency signal which is measured while going through each of the frequency ramps corresponds to an ambiguity line in the d/v space. In the case of two frequency ramps having different slopes, two ambiguity lines result, which also have different slopes and therefore intersect in a point. Distance and relative velocity of the object are unambiguously determined by this intersection point.
However, if multiple objects are located in the detection range of a radar sensor, the number of the intersection points of the ambiguity lines exceeds the number of the objects, which in turn prevents unambiguous determination of velocity and distance of each object.
One possibility for also obtaining unambiguous results in the case of multiple objects in the detection range of a radar sensor includes increasing the number of the frequency ramps having different slopes within the cyclically repeated modulation sequence. For a number n of frequency ramps having different slopes, ideally neglecting possible measurement inaccuracies, all n ambiguity lines also meet in one point, which may be viewed as a true distance/velocity combination of the object with a probability which rises with n. The probability that n ambiguity lines meet in one point which does not correspond to a real object (accidental or incorrect intersection point) decreases with number n of frequency ramps. However, the number n may not be selected as arbitrarily high, since, on the one hand, the computing effort rises with the number of the frequency ramps and, on the other hand, the duration of a modulation sequence lengthens in such a way that the radar sensor no longer achieves sufficient time resolution. In practice, three or at most four frequency ramps are therefore typically used in a modulation sequence. In such a case, however, accidental intersection points may also occur, in particular if the number of objects in the detection range of a radar sensor and therefore the density of the peaks in the spectra of the intermediate frequency signal is high.
A method for operating a radar sensor is known from German Patent Application Publication DE 102 43 811 A1, in which to reduce incorrect recognition of objects, the results of successive modulation sequences are compared and an object is not viewed as a real object while the object is repeatedly identified at positions matching one another in the distance/velocity space. For this purpose, the position to be expected of a possible object recognized in a first modulation sequence is predetermined for a subsequent modulation sequence on the basis of the established velocity and the established distance in the d/v space. However, this method also does not preclude that an accidental intersection point of ambiguity lines in the second modulation sequence will be precisely at the precalculated location of a possible object identified in the first modulation sequence, which would therefore result in an identification of the possible object as a real object.
It is therefore an object of the present invention to provide a method for operating an FMCW radar sensor, which has the lowest possible incorrect recognition rate and good time resolution. It is a further object to provide a radar sensor capable of performing the method.
This object is achieved according to the present invention by a method for operating a radar sensor of the type described at the beginning, and a radar sensor in which the slope of at least one of the frequency ramps is established for a subsequent sequence in such a way that none of the expected positions of a possible object in the distance/velocity diagram is at an intersection point of precalculated ambiguity lines of the other possible objects.
In this way, an accidental intersection point of ambiguity lines in the distance/velocity diagram is also prevented from being identified as a real object in the case of a smaller number of frequency ramps per modulation sequence. In contrast to methods in which the number of the frequency ramps of the modulation sequence is increased, the probability of the occurrence of accidental intersection points is not reduced. Rather, accidental intersection points are prevented from occurring at the expected positions of possible objects. As a result, the accidental intersection points in the distance/velocity space change their position in an uncoordinated way from modulation sequence to modulation sequence, while in contrast intersection points which correspond to real objects continuously occupy a position matching the predetermined object behavior. Accidental intersection points may be reliably recognized in this way.
It should be understood that the features mentioned above and those yet to be explained below may be used not only in the particular combination given but also in other combinations or alone.
Example embodiments of the present invention are illustrated schematically in the Figures and are described below in more detail with reference to the Figures.